Tundishes and ladles are intermediate containment vessels utilized in processing metals and metal alloys. These vessels contain a permanent refractory lining material which has high temperature resistance. A material such as 50 to 70% Al.sub.2 O.sub.3 brick is often used to form the permanent refractory lining. Although permanent liner materials are highly temperature resistant, contact with molten metal and numerous cycles of heating and cooling during molten metal processing can degrade and necessitate frequent replacement of the permanent liner. Therefore, disposable liners formed of dry vibratable, trowellable, gunnable or sprayable refractory materials are formed upon the permanent liner of a tundish or other molten metal processing vessel permanent liner.
The disposable liner acts as a thermal and chemical barrier which protects the permanent liner and simultaneously maintains the temperature of the molten metal. After a tundish or ladle has been utilized for numerous cycles, the disposable liner is replaced. Since the disposable liner has a different coefficient of thermal expansion as compared to the permanent liner, "deskulling" or removal of the disposable liner should be easily accomplished. However, refractory compositions of the past utilized to form such disposable liners have often included alkali oxides such as, for example, Na.sub.2 O and K.sub.2 O. Heat penetration of the tundish or ladle can cause a reaction which occurs at the interface of the disposable and permanent linings. At this interface, the alkali metal oxides may react with the permanent liner thereby causing fusion of the two liners. Therefore, deskulling of the disposable liner becomes difficult, and portions of the permanent liner which fuse to the disposable liner can be removed during this operation. Furthermore, contamination of the permanent liner increases the coefficient of thermal expansion of the layer adjacent the disposable liner. The differences in expansion between this layer and the remainder of the permanent liner can result in sheet spalling of the permanent liner, thus leading to premature failure.
Basic refractor materials are commonly used as disposable lining materials. The term "basic" refers to the chemical behavior of these materials rather than the complexity of their composition. Basic refractory materials include magnesia, doloma, olivine, calcia or mixtures thereof.
In addition to the refractory aggregate, such conventional refractory compositions may also include various binders, density reducing materials and plasticizers. Furthermore, water is generally added to the composition in order to facilitate application by trowelling, gunning or spraying.
Sprayable refractory materials generally contain from about 20 to 30 weight percent water, thus enabling them to be pumped through a hose and to be sprayed when mixed with compressed air in a spray gun. Sprayable liners are well adapted for use in casting vessels such as hot tops, ladles, tundishes, troughs and pipes.
It has heretofore been observed that sprayable refractory materials can suffer from slumping, i.e., a condition in which a recently applied refractory material lacks the necessary adhesion to remain fixed to the surface to which it is applied.
Therefore, it would be highly advantageous to provide a sprayable refractory material for use as a disposable lining which provides satisfactory thermal insulation, ease of removal from the permanent liner and which provides a uniform layer without significant slumping of wet material. It would be further advantageous to provide a sprayable material which achieves the above advantages while also providing a density of less than about 100 pounds per cubic foot ("p.c.f.") while allowing a spray gun residence time of up to about 30-40 minutes without blockages. Moreover, if these materials could be essentially free of alkali oxides, the problem of contamination and/or premature deterioration of the permanent lining could be reduced.